A silicon single crystal wafer principally used as a wafer for manufacture of IC devices such as semiconductor integrated circuits is fabricated by slicing a silicon single crystal grown by the Czochralski method (hereafter referred to as the CZ method) and polishing the sliced wafer.
Various heat treatments are performed in a variety of processes depending on configuration of an IC device to be manufactured during the manufacture thereof. Possible contamination of the wafer with a heavy metal such as iron (Fe), Ni, and Cu during these heat treatment processes will lead to generation of defects and electrical levels near the surface of the semiconductor silicon substrate, resulting in deteriorated characteristics of the device.
In order to avoid this, the contaminant heavy metal must be removed by gettering (capturing) from the surface area of the wafer in which the device is to be formed. Among known methods of gettering, representative are an internal gettering (IG) method to capture heavy metals within the silicon wafer and an external gettering (EG) method to capture heavy metals on the rear side of the silicon wafer. This invention involves the internal gettering.
It has been known that the internal gettering ability of silicon wafers is related with a precipitation amount and precipitation state of oxygen precipitates.
Specifically, a silicon single crystal grown by the CZ method inevitably contains oxygen in the course of pull-up growth. When the oxygen-containing silicon wafer is subjected to a heat treatment, the contained oxygen will precipitate to form oxygen precipitates within the wafer. The oxygen precipitates thus formed has an effect of gettering contaminant heavy metals. In general, the internal gettering ability against the heavy metals is increased as the precipitation amount of the oxygen precipitates is increased.
However, if the precipitation amount is too high, the mechanical strength of the silicon wafer will be deteriorated, leading to more occurrence of warpage or the like during the device manufacturing processes. Accordingly, the higher oxygen precipitation amount does not necessarily imply a favorable result. The oxygen precipitation must be controlled according to the contamination status of the device and the heat treatment process so that a required amount of oxygen precipitation can be obtained.
Among the contaminant heavy metals, Ni and Cu have a particularly high diffusion rate within the silicon wafer. Therefore, these heavy metals can be gettered sufficiently effectively during a normal cooling process without the need of performing a heat treatment for capturing the heavy metals if a certain amount of oxygen precipitates is preliminarily precipitated within the wafer.
Patent Document 1 listed below describes an invention relating to measurement of internal gettering ability against Ni. Patent Document 2 listed below also describes an invention relating to heat treatments performed to provide the internal gettering ability against Cu based on simulation.
However, among the contaminant metals, iron (Fe) presents a lower diffusion rate within the silicon wafer in comparison with Ni and Cu, and is thus most difficult to getter. Moreover, iron (Fe) is a heavy metal which is the most typically contaminant during the device manufacturing processes.
In order to have Fe atoms captured by the oxygen precipitates, it is necessary to design a heat treatment process such that the internal gettering is performed effectively, or to sufficiently increase the precipitation amount of oxygen precipitates. It is therefore crucial to have correct understanding of the relation between the internal gettering ability against iron (Fe) and the precipitation amount and precipitation state of the oxygen precipitates.
In Non-Patent Document 1 list below, Gilles et al. for the first time formulated the relation between the internal gettering ability against iron (Fe) and the precipitation state of oxygen precipitates, using a model (hereafter referred to as the Gilles model) as described below, and proved the effectiveness of their method through experiments.
In the first place, they derived an expression representing the internal gettering ability in the following manner.
They assumed that supersaturated Fe atoms would precipitate on the surface of spherical oxygen precipitates with a radius R within a silicon wafer. They also assumed that the concentration of iron (Fe) on the surface of the oxygen precipitates was equivalent to the solubility of iron (Fe). Based on the assumptions, a diffusion flux J (atoms/s) of Fe atoms to a single oxygen precipitate can be represented by the following expression (1) through regular expression development.J=4πRD(C(t)−Ceq)  (1)
In the expression (1), D denotes a diffusion coefficient (cm2/s) of iron (Fe) and is represented as D=1.3×10−3 exp(−0.68 eV/kbT). C(t) denotes a concentration (atoms/cm3) of iron (Fe) within a silicon wafer at time t. Ceq denotes a solubility (atoms/cm3) of iron (Fe) and is represented as Ceq=4.3×1022 exp(−2.1 eV/kbT). kb denotes a Boltzmann's constant and is represented as kb=8.6257×10−5 (eV/K). T denotes absolute temperature (K).
When the oxygen precipitates density is N (units/cm3), the time change ∂C(t)/∂t of the iron (Fe) concentration C(t) in the wafer can be represented by the following expression (2).∂C(t)/∂t=−4πRD(C(t)−Ceq)N  (2)
It was assumed here that the initial contamination concentration of iron (Fe) represented by Cini (atoms/cm3) would attenuate to the solubility of iron (Fe) after infinite time. Therefore, C(t)=Cini at t=0, and C(t)=Ceq at t=∞. Based on these conditions, the solution of the expression (2) is represented by the following expressions (3) and (4).(C(t)−Ceq)/(Cini−Ceq)=exp(−t/τ)  (3)1/τ=4πRDN  (4)
In these expressions, τ is a time which is required for the iron (Fe) concentration normalized in the form, for example of the left side of the expression (3) to become 1/e (e=base of natural logarithm, or 2.718) as a result of gettering, and is referred to as relaxation time. 1/τ corresponds to a gettering rate.
Gilles et al. conducted experiments under conditions where Cini was up to 1015 (atoms/cm3), and the temperature during internal gettering was set to between 200 and 300° C., and confirmed that the radius R and density N of the oxygen precipitates dominated the gettering rate (1/τ) in the form as represented by the expressions (3) and (4).
The Gilles model indicates that the time change of the iron (Fe) concentration normalized in the form for example of the left side of the expression (3) remains same regardless of the initial concentration Cini. This is commonly observed in diffusion phenomena.
Further, while the expressions (3) and (4) are obtained as solution of the expression (2) with the temperature being fixed, the solution can be obtained also for an actual heat treatment process involving temperature change by directly solving the expression (2) using the finite difference method, and calculating the change from the initial value.
However, there have been reported two facts that cannot be explained based on the Gilles model.
One of the facts is reported by Tobe et al., in Patent Documents 3 and 4 and Non-Patent Document 2 listed below.
They found, as a result of experiments, that when the initial contamination concentration was varied from 8×1012 (atoms/cm3) to 2×1014 (atoms/cm3), the iron (Fe) concentration normalized as in the left-hand side of the expression (3) changed significantly in dependence on the initial contamination concentration Cini. Specifically, they found 1/τ representing the gettering rate in the expression (3) varied significantly in dependence on the initial contamination concentration Cini of iron (Fe). It is difficult to explain the phenomenon found by Tobe et al. by using the Gilles model in which the relaxation time τ does not depend on the initial iron (Fe) concentration Cini.
Tobe et al. therefore proposed their own model (hereafter referred to as the Tobe model) in Patent Documents 3 and 4 to explain the dependency of the relaxation time τ on the initial contamination concentration. Specifically, the Tobe model assumes spheres of iron silicide (contaminant heavy metal silicide; FeSi2) composed of a number of iron (Fe) atoms obtained by dividing the total number of supersaturated iron (Fe) atoms in the silicon wafer by the number of oxygen precipitates. They assert that the dependency of the expression (3) on the initial contamination concentration can be explained by assigning the radius of the spheres to the radius R of the oxygen precipitates in the expression (4). Although it is not clearly known what physical meaning this model has, it is reported that the experiment results can be explained sufficiently based on this assumption.
However, the Tobe model does not consider at all the effects of the radius R of the oxygen precipitates on the internal gettering ability. Specifically, in the Tobe model, the radius R of the oxygen precipitates is deleted by substituting the radius R of the oxygen precipitates with the radius of iron silicide alone. Therefore, according to the Tobe model, the radius R of the oxygen precipitates does not contribute in any way to the result of prediction of the internal gettering behavior.
It is well known, however, that the size (radius R) of oxygen precipitates as well as the density N thereof has a strong effect on the internal gettering ability against iron.
Tobe et al. proposed, in Patent Document 5 listed below, a method of evaluating the radius R of the oxygen precipitates from the measurement result of the iron (Fe) concentration by creating a calibration curve is to represent the relation between the radius R of oxygen precipitates and the iron (Fe) concentration after a gettering heat treatment. This method was devised by focusing on the fact that the change in concentration of iron (Fe) due to gettering was sensitive to the radius R of the oxygen precipitates.
Nevertheless, Patent Documents 3 and 4 make no consideration to the effect given by the radius R of the oxygen precipitates to the internal gettering ability. It is therefore believed that the prediction of internal gettering behavior is possible only under a specific condition. However, it cannot be known from Patent Documents 3 and 4 what range of the radius R of the oxygen precipitates corresponds to such condition.
Patent Document 5 also only shows, by way of experiments, the relation between the iron (Fe) concentration after a heat treatment and the density N and radius R of oxygen precipitates under a specific condition that heat treatment is conducted at 600° C. for 20 minutes when the initial iron (Fe) contamination concentration is 1013 (atoms/cm3). Thus, in Patent Document 5, the relation among the internal gettering ability, the initial iron (Fe) contamination concentration Cini, and the density N and radius R of the oxygen precipitates remains unclear.
The other fact that cannot be explained by the Gilles model is reported by Hieslmair et al. in Non-Patent Document 3 listed below.
Specifically, Hieslmair et al. conducted experiments while variously changing the density N of oxygen precipitates and the heat treatment temperature during internal gettering. The initial concentration of iron (Fe) was in the range of 2 to 4×1013 (atoms/cm3). They found relation between the gettering heat treatment time and the iron (Fe) concentration, and fit the found relation to the expression (3) to obtain the relaxation time τ in the gettering reaction. Using the relaxation time τ thus obtained, they found the density at the effective gettering site by using the expression (4). As a result, when the density N of the oxygen precipitates was 10109 (units/cm3) or lower, the effective gettering site density did not depend on temperature and was substantially identical with the density N of the oxygen precipitates. However, when the density N of the oxygen precipitates was 10109 (units/cm3) or higher, it was found that that the ratio of the density N of the oxygen precipitates to the effective gettering site density decreased remarkably as the heat treatment temperature became higher. The gettering of iron (Fe) is believed to be most effective in a range of heat treatment temperature between 600 and 700° C. Further, it is generally believed that the density N of the oxygen precipitates effective to the internal gettering is 109 (units/cm3) or higher. Hieslmair et al. report that when experiments are conducted under these conditions, the ratio obtained by dividing the effective gettering site density by the density N of the oxygen precipitates is reduced to near 1/100. This means that the expressions (3) and (4) of the Gilles model do not work at all in a practical range of gettering heat treatment temperature and or in a practical range of of the density N of the oxygen precipitates.
As described above, conventionally, the internal gettering behavior in a silicon substrate could be predicted substantially correctly only under limited conditions, but correct prediction of the behavior was not possible under practical conditions of gettering temperature and oxygen precipitates density. Moreover, there was even no means for predicting the behavior based on the initial contamination concentration.
[Patent Document 1]
    Japanese Patent Application Laid-Open No. 2004-31845[Patent Document 2]    Japanese Patent Application Laid-Open No. 2003-318181[Patent Document 3]    Japanese Patent Application Laid-Open No. H11-283986[Patent Document 4]    Japanese Patent Application Laid-Open No. 2003-282576[Patent Document 5]    Japanese Patent Application Laid-Open No. 2003-257983[Non-Patent Document 1]    D. Gilles, and E. R. Weber: Physical Review Letters, Vol. 64, No. 2 (1990), p 196[Non-Patent Document 2]    Tobe, Hirano, and Hayamizu: The Japan Society of Applied Physics, Silicon Technology, No. 5, 1998, p 44[Non-Patent Document 3]    H. Hieslmair, A. A. Istratov, S. A. McHugo, C. Flink and E. R. Weber: J. Electrochemical Society, Vol. 145, No. 12 (1998) p 4259